Cross-flow ion mobility analyzer

ABSTRACT

A cross-flow ion mobility analyzer (CIMA) that includes a component of gas flow that opposes an electric field that is established within a channel, wherein ions are carried through the channel, wherein ions of a specific mobility are trapped by the opposing electric field and flow field within the channel and are detected when the ions reach the end of the channel, wherein a detector at the end of the channel sees a continuous stream of mobility-selected ions, and wherein different ions are selected by modifying the electric field and/or the velocity of the flow field.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims priority to provisional patent application Ser.No. 60/461,890, filed Apr. 9, 2003.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to separation, storage, and analysis ofions according to ion mobilities of charged particles and chargedparticles derived from atoms, molecules, particles, sub-atomic particlesand ions. More specifically, the present invention is an ion mobilityanalyzer that is employed to detect a wide range of chemicals, whereinthe analyzer differentiates chemical compounds based upon their ionmobilities.

2. Description of Related Art

To understand the advantages of the present invention, it is useful toexamine the state of the art of mass spectrometry. Chemical analysis ofcharged particles and charged particles derived from ions, molecules,particles, sub-atomic particles and atoms (hereinafter to becollectively referred to as ions) can be done by separating their ionicforms according to their mass-to-charge ratios. There are various kindsof mass spectrometers. Each mass spectrometer has been found to have itsown special characteristics and applications, as well as limitations. Inthe case of time-of-flight (TOF) mass spectrometry, the TOF massanalyzer measures the mass-to-charge $\left( \frac{m}{z} \right)$dependent time that it takes for ions of different mass-to-charge ratiosto move through a flight tube from an ion source to a detector. Theanalysis is based on measurements of the flight time required for theion to move along a tube of a defined length in an environment that isfree of electric fields.

Time-of-flight mass spectrometry performs its analysis based on thecharacteristics of charge and mass of ions. In contrast, a relatedtechnique known as ion mobility mass spectrometry (IMS) is dependentupon the charge, size and shape (the cross-sectional area) of moleculesto perform its analysis of ions.

IMS is a gas phase electrophoretic separation technique in which ionsare separated based on their ionic mobilities as the ions drift througha buffer gas under the influence of an electric field. The analysis isbased on measuring the drift time that it takes for the ion to movealong a drift tube of a defined length in an applied electric field.

There are different types of IMS instruments that need to be understoodin order to understand the principles of the present invention. Inconventional IMS, an electric field produces a linear relationshipbetween the drift velocity and electric field. Accordingly, reducedmobility is generally independent of the electric field. A sample isintroduced into an ionization region containing an ion source. Ionizedsamples are then accelerated into a drift region in a drift tube, oftenwith a buffer gas introduced from the opposite direction. Ions areseparated as they drift through this buffer gas. Separation of the ionsis based upon size, shape, and charge of the ions. Ions that driftthrough the buffer gas are registered at the detector. Conventional IMSsystems generally use a low electric field and are characterized byhaving a low duty cycle.

In differential mobility analysis (DMA), ions are separated according totheir mobilities by the application of an electric field and a flowfield that are orthogonal to each other. The ions of differentmobilities are dispersed in space so that only ions of a selectedmobility pass through a detector slit. DMA is often used in aerosolexperiments to analyze particles of a given size.

The last type of mobility analyzer is known as high-field asymmetricwaveform ion mobility spectrometry, or FAIMS. In FAIMS, two concentrictubes or plates are generally used. A high electric field is applied fora short time, and then a low electric field is applied for a longerduration, with the average applied electric field being balanced. Thenon-linearity of the FAIMS system is generally attributed to thedifferent cross-sectional areas of the ions that are drifting throughthe tube or between the plates. Accordingly, the method takes advantageof the different mobilities of ions in a high electric field as comparedto a low electric field.

Another way of describing FAIMS is to say that the separation of ions isbased on the nonlinear dependence of the mobility constant with respectto the electric field intensity. The change in mobility at high electricfield values appears to reflect the size of the ion, its interactionwith the buffer gas, and the structural rigidity of the ion. Thus, theratio of high electric field mobility to low electric field mobility isused in the characterization of ions in FAIMS.

IMS as described by the three techniques above is a relatively fastmethod of ion analysis, is highly sensitive, moderately selective, andhas a low limit of detection. However, IMS has generally received littleattention because of its relatively poor resolution, limited lineardynamic range, and the previously mentioned low to moderate selectivity.

Accordingly, what is needed is a new form of ion mobility spectrometrythat overcomes the disadvantages of the existing IMS methods.Specifically, it would be an advantage over the state of the art of IMSto be able to provide increased sensitivity, increased resolution, moreaccurate mobilities and specific detection.

BRIEF SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method of ionmobility spectrometry that is more sensitive in particular modes ofoperation than existing IMS methods.

It is another object of the present invention to provide a method of ionmobility spectrometry that has increased resolution as compared toexisting IMS methods.

In a preferred embodiment, the present invention is a cross-flow ionmobility analyzer (CIMA) that includes a component of gas flow thatopposes an electric field that is established within a channel, whereinions are carried through the channel, wherein ions of a specificmobility are trapped by the opposing electric field and flow fieldwithin the channel and are detected when the ions reach the end of thechannel, wherein a detector at the end of the channel sees a continuousstream of mobility-selected ions, and wherein different ions areselected by modifying the electric field and/or the velocity of the flowfield.

These and other objects, features, advantages and alternative aspects ofthe present invention will become apparent to those skilled in the artfrom a consideration of the following detailed description taken incombination with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 is a schematic drawing of the first embodiment of a cross-flowion mobility spectrometer comprised of concentric cylinders aselectrodes of the system.

FIG. 2 is an illustration of fluid flow in relation to the electrodesand the electric field that is disposed between them.

FIG. 3 is a graph describing an injection and transmission profile fromthe cross-flow ion mobility system.

FIG. 4 is a graph describing a decay profile from the cross-flow ionmobility system.

FIG. 5 is a graph describing an ion spectrum from the cross-flow ionmobility system.

DETAILED DESCRIPTION OF THE INVENTION

Reference will now be made to the drawings in which the various elementsof the present invention will be given numerical designations and inwhich the invention will be discussed so as to enable one skilled in theart to make and use the invention. It is to be understood that thefollowing description is only exemplary of the principles of the presentinvention, and should not be viewed as narrowing the claims whichfollow.

The present invention is a cross-flow ion mobility analyzer (CIMA) thatis capable of detecting a wide range of chemicals includingpharmaceuticals, environmental pollutants, chemical and biologicalwarfare agents, agrichemicals, and petrochemicals. An immediate need forthis technology is airport screening for residues of explosives that maybe present on luggage, packages, and personnel. Another usefulapplication is characterizing sizes of lipo-proteins from blood samples.It will be understood by those skilled in the art that there are otherapplications for a CIMA of the present invention.

Similar to IMS, the present invention is based on the mobilities of ionscreated from target analytes. However, the new cross-flow ion mobilityanalyzer technology uses a modified principle of operation todifferentiate compounds based on their ion mobilities, and promisesenhanced sensitivity and greatly enhanced resolution over all previousimplementations of IMS, including conventional, differential, and FAIMS.The present invention results in more accurate and specific detection ofchemicals.

The present invention is illustrated in a first embodiment in FIG. 1.FIG. 1 is a cross-sectional schematic diagram of the CIMA device 10. TheCIMA device includes a drift region (i.e. a cross-flow region) that isformed by the gap or space 16 between two concentric metal cylinders 12,14 that are approximately 6 inches long. It should be understood thatthis first embodiment that uses cylinders is for illustration purposesonly.

Alternative embodiments of the present invention include replacing thetwo concentric cylinders 12, 14 with electrodes that can create anelectric field between them. For example, concentric spheres, a seriesof stacked electrodes, substantially parallel plates, and non-parallelplates can also be used. One of the distinct advantages of the firstembodiment of concentric cylinders 12, 14 is to minimize edge effects onlaminar flow and electric fields.

Returning to the first embodiment, the inner cylinder 12 has a diameterof 2 inches and the outer cylinder 14 has a diameter of 4 inches,thereby leaving a one inch gap 16 between the cylinders 12, 14. Themiddle 4-inch lengths of both cylinders 12, 14 include holes 18(approximately 10,000 each) that are approximately 0.0128 inches indiameter. The thickness of the cylinders 12, 14 is approximately 0.25inches. A front end of the cylinders 12, 14 include a hemispherical endcap 20. The end cap 20 serves at least the function of delivering ionsto the gas cross-flow region 16 between the cylinders 12, 14. An inletaperture 22 through the end cap 20 functions as an inlet for the ionsthat are to be separated by their mobilities in the gas cross-flowregion 16.

The inlet aperture 22 in the end cap 20 will typically be coupled tosome ion source (not shown). It should be understood that anyappropriate atmospheric ionization techniques can be used to create ionsfor delivery to the CIMA device 10. The following is a list of somecommonly used ionization techniques: electron impact, chemicalionization, fast ion or atom bombardment, field desorption, laserdesorption, plasma desorption, thermospray, electrospray,photoionization, inductively coupled plasma, and atmospheric pressureionization. This list should be considered as representative only, andis not intended to exclude other appropriate ionization systems that mayalso be used with the CIMA device 10 of the present invention.

In one alternative embodiment, an ionization chamber is disposeddirectly adjacent to the aperture inlet 22 where samples can be ionizedimmediately before being drawn into the cylinders 12, 14.

The CIMA device 10 is housed in an enclosure or housing 24 that issealed to thereby maintain the appropriate pressure and constant gasflow that is needed for operation of the present invention which willnow be explained.

In operation of the first embodiment, the housing 24 is first purged ofair and bathed in nitrogen gas. Both the inner and outer cylinders 12,14 are coupled to at least two voltage sources (if ground is considereda voltage source) (not shown) so that both cylinders 12, 14 function aselectrodes. The cylinders 12, 14 are set at different potentials tothereby generate a potential between the first cylinder 12 and thesecond cylinder 14.

In the example configuration shown in FIG. 1, the desired range forelectrical potentials will generally vary from hundreds up to thousandsof volts. However, it should be remembered that for whatever size ofelectric field that is established between the cylinders 12, 14, therewill be an opposing gas flow that must be sufficiently strong enough tocreate a balancing effect. Nevertheless, it is possible to increase ordecrease the electrical Potential and the opposing fluid flow dependingupon the desired performance of the present invention.

The significance of the fact that the present invention operates usingelectric potentials in the hundreds and perhaps thousands of volts isworth noting. One implication of this fact is that the present inventionoperates with voltages that are much easier to operate with than thetens of thousands of volts that are part of at least conventional andFAIMS IMS systems.

Along with the electric field that is established in the cross-flowregion 16 between the cylinders 12, 14, a critical aspect of the presentinvention is the creation of a cross-flow of gas that opposes theelectric field. A velocity of the gas cross-flow is therefore set to anyappropriate value as known to those skilled in the art. The gascross-flow is shown in FIG. 1 as being created by a flow of a gas intothe first cylinder 12 that is directed outwards through the holes 18into the cross-flow region 16, and then through the holes 18 in thesecond cylinder 14 into a space 26 in the housing 24. This gascross-flow is represented by lines 28. FIG. 1 indicates that a venturiair device 30 directs the gas cross-flow into the first cylinder 12. Anexhaust-aperture 32 is also shown in the housing 24.

Some aspects of the present invention that can be explained at thispoint include the fact that the housing 24 can operate at normalatmospheric pressure, at an elevated pressure, or at a reduced pressure,depending upon the desired performance of the CIMA device 10.Furthermore, it is possible to operate certain regions of the CIMAdevice 10 at one pressure, and a different region at a differentpressure. For example, the gas cross-flow region 16 may be at a firstpressure, and the ions may be drawn into a detection area that operatesat a different second pressure.

Another aspect of the present invention that bears explanation is thatit is recognized that the gas cross-flow velocity decreases as the gasmoves from the inner cylinder 12 to the outer cylinder 14. What is notimmediately apparent is that the strength of the electric field changesin the same proportion as the gas cross-flow velocity. Accordingly, abalanced condition is maintained in the gas cross-flow region 16 betweenthe cylinders 12, 14 regardless of the position of the ions of selectedmobilities.

Related to this concept of having a balanced condition is the concept ofa fastscan. The balance condition needs to remain the same as an iontravels through the CIMA instrument 10. However, as the CIMA instrument10 is used to perform a scan, an ion that is in balance when it is at abeginning point of the gas cross-flow region 16 will be out of balancewhen it arrives at an exit point because the electric field changes. Forexample, when scanning from a low voltage to a high voltage, the voltageis too high by the time the ion reaches the exit point, so it is out ofbalance and is ejected.

A solution to this problem in the present invention is to arrangeelectrodes (the cylinders 12, 14) so that at any given instant, theelectric field for an ion near the exit point is less than for an ionnear the entry point, assuming that it is possible to perform a scanfrom low to high voltage. It should be apparent that it is possible tosimply reverse the process if scanning is in the opposite direction(i.e. from a high voltage to a low voltage). One method of performingthis action is to break the electrodes into electrically isolatedsections and apply voltages separately to each section. Another way isto simply make the electrodes non-parallel. The end result of eitherscheme is to create a desired voltage gradient to compensate forcreating an unbalanced situation.

Another solution is to make the gas cross-flow velocity lower at theexit point than at the entry point. This assumes that the system isbeing used to scan from low to high voltage. Again it should be apparentthat it is possible to simply reverse the process if scanning is in theopposite direction (i.e. from a high voltage to a low voltage).

In a final alternative, it is possible to combine the methods ofcreating a desired voltage gradient with the change in gas cross-flowvelocity.

With the operation of the CIMA device 10 as described above, wherein anelectric field is generated between two cylinders 12, 14, and anopposing gas cross-flow is created in the same region, the presentinvention is capable of operating in a trapping mode. In other words,ions will be trapped within the gas cross-flow region 16 if there is noforce to move them out. This trapping mode may be useful for gatheringions that can then be delivered from the was cross-flow region 16, forexample, in a pulsed mode.

Nevertheless, in an alternative embodiment, the present invention alsoincludes the creation of an axial gas flow which is represented by lines36. The axial gas flow also operates within the gas cross-flow region16, but operates independently of the gas cross-flow, and flows along along axis of the cylinders 12, 14, substantially perpendicular to thegas cross-flow. However, the net gas flow is the combination of theaxial gas flow and the gas cross-flow.

It is observed that the axial gas flow may affect the resolution of theCIMA device 10. The slower the axial gas flow, the higher the predictedresolution.

In essence, when ions are injected into the system at the sample inletaperture 22, the axial gas flow draws the ions into and then carriesthem through the cross-flow region 16 where the electric field appliedin one direction and gas cross-flow in an opposing direction is used toseparate the ions based on their mobilities it should be mentioned thatwhile a vacuum pump may be used to draw the ions into the gas cross-flowregion 16, other systems can be used to cause this effect. The gascross-flow assists in separating and suspending ions in space in the gascross-flow region 16.

When the CIMA device 10 is functioning as a detection device, a detector34 is generally going to be disposed at the end of the gas cross-flowregion 16. The detector 34 is maintained at a zero potential so thations leaving the high electric field of the gas cross-flow region 16will enter a controlled-voltage zone, and collapse without interferenceinto the detector where they are registered.

It is noted that the first embodiment does not include any sort of slitin front of the detector 34. However, it is observed that an aperturecould be disposed between the gas cross-flow region 16 and the detector34 that could lead to a different detector or detection system. Theaperture can be comprised of a slit, a plurality of slits, holes, etc.

Regarding a gas that is used in the CIMA device 10, it was mentionedthat nitrogen is used as the gas cross-flow. Nitrogen was selectedbecause of its inert nature. Other inert gases can be used. However, inanother aspect of the present invention, modifying gases could also beselected. A modifying gas is a gas in which the transport properties ofions are different from those referred to previously.

Another aspect of the present invention that bears discussion is theaffect of temperature on performance. It may be that ion mobilities maybe affected by the temperature at which the ions are being separated.Accordingly, ion mobilities may be increased or decreased depending uponwhether or not temperature of the ions is decreased or increased.

In the present invention, it was previously mentioned that it ispossible to suspend ions of a given mobility in the gas cross-flowregion 16. The amount of time that the ions can remain suspended islimited by diffusion to walls of the cylinders 12, 14. It is noted thatthe term “diffusion” as used here is meant to refer to turbulent anddispersive diffusion as understood by those skilled in the art. FIG. 2is provided as an illustration of how this is achieved. It is noted thatthis is in the context of also having a changeable electric field or gascross-flow.

In the CIMA device 10 illustrated in the first embodiment, FIG. 2illustrates that the effort is directed to making F₃ equal F₄ to therebyobtain suspension of the ions in the gas cross-flow region 16. However,diffusion of ions toward the walls of the cylinder 12, 14 can be limitedor reduced by some type of focusing method.

Referring to FIG. 2, mass conservation requires that the total net flowof the flows F1 through F4 into or out of the device is zero. Thus, anythree of the flows may be independently controlled. In this process, thegas flow F1 must be into the CIMA device 10 or the ions will beprevented from entering the CIMA device 10.

There are at least two ways in which focusing can be achieved. The firstfocusing method for the present invention is to make F₃ greater than F₄.This action pushes ions toward the opposite cylinder wall. To suspendthe ion in the gap (cross-flow region) requires that the electric fieldbe slightly increased. This combination produces a focusing effect. Thepoint of focus can be moved by adjusting the electric field or the gascross-flow. Ions of a given mobility range (a passband) are focusedwithin the gap, and ions outside of the range are pushed to one wall orthe other, and will not reach the detector 24. Each mobility value thatis focused within the gas cross-flow region 16 is focused on a differentposition within the gap.

The second focusing method for the present invention is achieved bysuperimposing a small parabolic potential over the electric field. Theparabolic potential is a perturbation of the electric field opposing thegas cross-flow. Adding this electric field is a close analogy tomodifying the flow field in the previous paragraph. However, it shouldbe understood that while focusing decreases the resolution of the CIMAdevice 10, it increases transmission efficiency.

The following material explains the principles upon which the presentinvention is based, as well as some simulation results that confirmoperation as desired. In the new CIMA device 10 of the presentinvention, ions are separated in a region where two opposing forces acton the ion: an electric field in one direction and a buffer gas flow(gas cross-flow) in the opposite direction. Ions are suspended andseparated in the center of an annular region (gas cross-flow region 16).

Unlike IMS techniques where ions drift from one end of a cylinder to theother and are separated, separation occurs in the CIMA device 10 of thepresent invention as ions that are suspended from each other between thecylinder walls of the annular region (separation in one direction) andmove from injection point to detector (separation in second direction,i.e. “axial direction”) by the weak axial gas flow. Alternatively, aweak axial electric field can be superimposed on the electric field.However, it has been determined that it is easier to generate the axialgas flow rather than the weak axial field.

The following equation is representation of the distribution of the ionsacross the gas cross-flow region 16. This first equation is thereforesimply a representation of a calculation that can be performed thatrepresents a simplified model of one aspect of the CIMA device 10. Thesimplified model is represented by the equation${\frac{\partial C}{\partial t} = {{D\quad\frac{\partial^{2}C}{\partial x^{2}}} - {V\quad\frac{\partial C}{\partial x}}}},$where D=diffusion coefficient, C=concentration of analyte, V=flowvelocity which is a sum of v_(g) and v_(d), v_(g)=velocity of gas (gascross-flow velocity), and v_(d)=velocity of drift (caused by electricfield). It should be understood that v_(d) is a function of the mobilityand electric field (i.e. v_(d)=KE, where K is the mobility constant andE is electric field strength). D is related to the mobility by theequation ${K = \frac{eD}{kT}},$(assuming normal or dispersive diffusion, and not turbulent diffusion)where K=the mobility constant, e=ionic charge, k=Boltzmann constant,T=gas temperature, and D=diffusion coefficient.

Using appropriate units (K in cm² V⁻¹s⁻¹, D in cm² s⁻¹, and T inKelvin), $K = {1.1605 \times 10^{4}{\frac{D}{T}.}}$Solving the differential equation gives two trial solutions, a “cosine”and “sine” solutions, which are${F\left( {x,t} \right)} = {{\exp\left( {\left( {{- \frac{V^{2}}{2D}} - {\frac{4D\quad\pi^{2}}{L^{2}}\left( {n - 0.5} \right)^{2}}} \right)t} \right)}{\exp\left( {\frac{V}{2D}x} \right)}{\cos\left( {2{\pi\left( \frac{n - 0.5}{L} \right)}x} \right)}}$and${{G\left( {x,t} \right)} = {{\exp\left( {\left( {{- \frac{V^{2}}{2D}} - {\frac{4D\quad\pi^{2\quad}}{L^{2}}n^{2}}} \right)t} \right)}{\exp\left( {\frac{V}{2D}x} \right)}{\sin\left( {2\pi\quad\frac{n}{L}x} \right)}}},$where n=the number of half waveforms between the boundary. At boundaryconditions, the concentration becomes zero as ions are lost when theycollide at the walls of the container. Within the space between thewalls (i.e. the gas cross-flow region 16) the total ion concentration isexpressed as${C\left( {x,t} \right)} = {{\sum\limits_{n = 1}^{\infty}{A_{n}{F_{n}\left( {x,t} \right)}}} + {B_{n}{{G_{n}\left( {x,t} \right)}.}}}$

How much of the initial concentration makes it to the detector 34 is ameasure of the transmission efficiency of this method. The resolution ishow well sample ions are separated between the walls and along the path(gas cross-flow region 16) between cylinders 12, 14 to the detector 34as compared to ions of slightly different mobilities.

The CIMA device 10 of the present invention can be compared to FAIMS inwhich the gas cross-flow is likened to the asymmetric AC voltage andchanging static electric field compared to compensation voltage inFAIMS. In a theoretical study, a 1,000 V/cm potential was applied to a10 cm tube to achieve a resolution of 1400. The suspending effect of theopposing gas flow is what actually makes CIMA a more selective andresolving method.

Preliminary work was done by developing a mathematical model for CIMA,and then performing simulations for the proof of principle of the CIMAconcept. Several injection profiles were simulated using a betadistribution function. This beta distribution function is given as${{C\left( {x,{t = 0}} \right)} = {\left( {\frac{1}{2} + \frac{x}{L}} \right)^{\alpha - 1}\left( {\frac{1}{2} - \frac{x}{L}} \right)^{\beta - 1}\frac{1}{L}\frac{\gamma\left( {\alpha + \beta} \right)}{{\gamma(\alpha)}{\gamma(\beta)}}}},$where α>0, β>0. γ(α+β) is the gamma function defined asγ(α) = ∫₀^(∞)x^(α − 1)𝕖^(−x)𝕕x.

For example, consider the following injection and transmission profilesin FIG. 3, depending on the alpha and beta values chosen. At t=0, when asample is injected into a CIMA device, it is possible to model theconcentration to fit into any of the profiles in FIG. 1, andC(x, t = 0) = ∑A_(n)F_(n)(x, t = 0) + ∑B_(n)G_(n)(x, t = 0).The equation above is solved by first finding all values of A_(n) andB_(n), which when summed together reproduce the original profile. Fromthe equation above, the solutions for A_(n) and B_(n) are given asfollows:${A_{k} = \frac{\int_{{- L}/2}^{L/2}{{C\left( {x,{t = 0}} \right)}{\exp\left( {{- \frac{V}{2D}}x} \right)}{{Cos}\left( {2{\pi\left( \frac{k - 0.5}{L} \right)}x} \right)}}}{\int_{{- L}/2}^{L/2}{{Cos}^{2}\left( {2\pi\quad\frac{k - 0.5}{L}x} \right)}}},{and}$$B_{k} = \frac{\int_{{- L}/2}^{L/2}{{C\left( {x,{t = 0}} \right)}{\exp\left( {{- \frac{V}{2D}}x} \right)}{{Sin}\left( {2\pi\quad\frac{k}{L}x} \right)}}}{\int_{{- L}/2}^{L/2}{{Sin}^{2}\left( {2\pi\quad\frac{k}{L}x} \right)}}$respectively. Where k takes values from 1 to infinity.

A computer program was used for statistical modeling to perform thesimulation. The injection profile was simulated to determine an optimalentrance slit, then a decay profile that reproduces the concentrationdecay as the analyte moves from the injection port to the detector, andfinally the detector spectral output. In an actual simulation, theseries is truncated to something less than infinity for practicalpurposes.

The following parameters were used for the simulation: a drift cylinderof 10 cm, a plate gap of 3 cm, flight time from injection to detector of1 second, an axial length of 10 cm, a gas cross-flow velocity of 10 m/sand a voltage of 10 kV. The following compounds with their K₀ were used:methylamine, 2.65; ethylamine, 2.36; formamide, 2.45; dimethylamine,2.46; and isopropylamine, 2.20; in reduced mobility units of cm²/V/s.

In the simulation, an equal amount of the five samples was injected intothe system. These plots show some very interesting and unexpectedresults. The first figure shows transmission of analytes within opposingfields, namely; the gas cross-flow and an opposing electric field. FIG.3 shows how most of the sample distribution is centered in the gascross-flow region 16.

FIG. 4 shows a typical decay profile in one second. It is interesting tonote that there is a transmission efficiency of about 99%. Thistransmission efficiency shows that most of the injected sample is notlost to the wall, but that it reaches the detector.

FIG. 5 shows the detector output in a scanned electric field (i.e.scanned mobility spectrum). Calculation of resolution gives a value of1400 full-width at half peak maximum.

The following generalizations are drawn from the simulation results: aresolution greater than the resolution value predicted or achieved bytraditional IMS methods. Enhanced selectivity is possible as thesimulation shows that compounds with reduced mobility difference of 0.01unit can be separated. The transmission efficiency shows that CIMA willimprove detection limits when operated in a selected mobility operatingmode as compared to the levels currently achieved by the traditional IMSmethods, as less than 1% of the initial sample is lost.

In an alternative aspect of the present invention, it should beunderstood that although the present invention is a gas phase device,the present invention can also operate with liquids. For example, incomparison to capillary electrophoresis, there is virtually noelectro-osmotic flow. Furthermore, the present invention is a continuousrather than a pulsed system that is useful for preparative as well asanalytical separations.

Regarding detectors, the present invention can use detectors based onFaraday detection, electron multiplier, multi-channel plate,charge-coupled detectors, an array detector, or any other detectionmethod including mass spectrometry or IMS (with or without collisioncells). The analyzer and detector can separate and detect eitherpositive or negative ions separately, or both positive and negative ionsat the same time using parallel cross-flow channels.

The present invention can be operated so as to monitor a single ion, orseveral selected ions at the same time, scanned through a predeterminedmobility range, or with an array detector to monitor all ionssimultaneously separated in space.

An additional aspect of the present invention includes the ability toprovide additional selectivity by applying an asymmetric electricwaveform between the electrodes of the CIMA device 10, similar to whatis practiced in FAIMS. However, no DC field is needed because the gasflow takes the place of the DC field. In addition, the present inventioncan be configured such that one half of the CIMA device 10 operates witha direct DC potential between the electrodes, and the other halfoperates with a high field asymmetric waveform potential between theelectrodes.

Furthermore, the asymmetric waveform could also be superimposed on a DCpotential.

It is to be understood that the above-described arrangements are onlyillustrative of the application of the principles of the presentinvention. Numerous modifications and alternative arrangements may bedevised by those skilled in the art without departing from the spiritand scope of the present invention. The appended claims are intended tocover such modifications and arrangements.

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 66. (canceled)
 67. A method for using a cross-flow of fluidthat opposes an electric field to thereby separate ions according to ionmobilities of charged particles and charged particles derived fromatoms, molecules, particles, sub-atomic particles and ions, said methodcomprised of: (1) establishing an electric field in a channel of across-flow ion mobility analyzer that is substantially perpendicular toa direction of travel of ions; (2) establishing a first fluid flow thatopposes the electric field in the channel and which is substantiallyperpendicular to the direction of travel of ions; (3) separating ions ofa specific mobility according to a strength of the electric field and arate of the gas flow; (4) providing a detector at the end of thechannel; and (5) detecting ions that arrive at the detector.
 68. Themethod as defined in claim 67 wherein the method further comprises thestep of creating the electric field using at least two electrodes. 69.The method as defined in claim 68 wherein the method further comprisesthe step of making the at least two electrodes at least partiallypermeable to the fluid flow to thereby enable the creation of a fluidcross-flow in the channel.
 70. The method as defined in claim 67 whereinthe method further comprises the step of providing a second fluid flowthat is substantially perpendicular to the first fluid flow, and whereinthe second fluid flow is generally parallel to a long axis of thechannel, wherein the second fluid flow delivers the ions to thedetector.